Information recording medium having a glass substrate

ABSTRACT

An information recording disk for recording an information signal along a track defined by a depression on the disk as a change of physical property of a recording material deposited on the track comprises a disk-shaped glass substrate made of a glass and carries a groove corresponding to the track. The groove has a surface roughness substantially smaller as compared to the surface roughness caused at a bottom surface of a groove on a silica glass substrate when both the disk-shaped glass substrate and silica glass substrate are dry-etched under same conditions. The disk-shaped glass substrate comprises SiO 2  component and one or both of Al 2  O 3  and BaO components with substantially no alkali components. Further, a recording layer comprised of the recording material which changes in physical property responsive to a projected energy beam is deposited on the substrate such that the depression is formed in correspondence with the track.

BACKGROUND OF THE INVENTION

The present invention generally relates to information signal recordingmedia and in particular to a disk-shaped information recording medium onwhich an information signal is recorded by means of an energy beam suchas a laser beam (optical beam) or an electron beam.

Optical information recording medium such as an optical disk or amagneto-optical disk hereinafter referred to as a disk is recorded withan information signal such as a video signal or an audio signalmodulating an optical beam which moves along a spiral or concentricguide groove provided on the surface of the disk. At the time ofreproduction, the guide groove is irradiated by an optical beam and theinformation signal is reproduced by processing the optical beamreflected back from the guide groove. For this purpose, tracking of thethe optical beam must be controlled such that the optical beam tracesthe guide groove properly. Such a tracking of the optical beam isachieved by means of a known servo control system which controls theoptical beam on the basis of the optical beam reflected back from theguide groove. The same tracking principle is known to be applicable torecording the information along a preformed guide track. Thus, the guidegroove is used not only for storage of information signals but also formaintaining a proper tracking of the optical beam at the time ofrecording and reproducing of the information signal on and from thedisk. The guide groove is usually a spiral-shaped or concentriccontinuous groove but may be a series of intermittent pits as in thecase of a reproducing only type optical disk such as a so-called CompactDisk.

The information recording medium having the guide groove or pits asaforementioned is manufactured by impressing a pattern on a metalstamper which is an inversion of the pattern of the groove or pits to beformed on the surface of the disk. The disk may be formed by injectionmolding or compression molding of a thermoplastic resin using the metalstamper as the mold. The manufacture of the disk by the injectionmolding or compression molding has a high productivity and is suited forautomatic production. On the other hand, the disk thus produced has aproblem in that the impression of the guide groove or pits by thestamper is not satisfactorily precise. Further, the disk tends to showbirefringence, and the disk is deformed by the moisture in the air.Thus, these problems of the conventional plastic disk will createdifficulties at the time of recording and reproducing.

In order to eliminate these problems, use of silica glass for thesubstrate of the disk is proposed. The use of silica glass as thesubstrate of the disk is advantageous in that the disk thus produced hasa small thermal expansion and shows virtually no absorption of moisture.Further such a disk has negligible birefringence. In order to providethe guide groove or pits on the surface of such silica glass disk, alayer of UV-cure resin which is a resin cured by ultraviolet radiationis deposited on the surface of the glass. In detail, a layer of UV-cureresin is first applied to the surface of the stamper in an uncured stateand the resin is covered by the silica glass disk so as to besandwitched between them. Next, an ultraviolet light is irradiated on tothe resin through the glass disk and the resin is cured. The stamper isthen removed and a two layered disk comprising a glass substrate and alayer of the cured resin carrying the pattern of groove or pits thereonis obtained. The disk thus produced is superior as compared with theplastic disk of thermoplastic resin in that a resin having a lowviscosity at room temperature can be used and the groove or pits on thestamper is transferred to the plastic layer more accurately as comparedto the case of the conventional disk molded from the usual thermoplasticresin. However, this procedure involves delicate steps of sandwitchinguncured resin layer as well as of the separation of the stamper from thecured resin layer which pauses difficulties in automatization of itsmanufacture.

The disk produced by molding of the thermoplastic resin or byapplication of the UV-cure resin on the silica glass substrate isfurther deposited with a reflection layer by vacuum vapor deposition orby sputtering. During such procedure, there is a problem that water isreleased from the cured resin or molded plastic due to the heating andthe structure and property of the reflection layer become deteriorated.

In order to eliminate this problem, provision of the guide groove orpits directly etched on the surface of the silica glass substrate isproposed in the U.S. Pat. No. 4,655,876 as well as in the JapaneseLaid-open Patent Application No. 26951/1986 in which the respectiveassignee and the applicant are same as the assignee of the presentapplication. According to the procedure proposed by the aforementionedpatent and patent application, a layer of photoresist is applied on apolished surface of a silica glass substrate. Then, the pattern of guidegroove or pit is written on this photoresist by means of a focused laserbeam. Then, after a development of the laser exposed photoresist, thesubstrate is subjected to a dry etching such as a plasma etching using aplasma gas such as CF₄. The plasma gas selectively reacts with thesilica of the glass and the silica material at the portion of the disknot covered with the photoresist is removed by the reaction. On theother hand, the portion of the silica masked by the photoresist is notsubjected to the reaction. The reaction is continued until an intendedgroove depth is reached. After the groove reaches the intended depth,the plasma gas is changed to a gas containing oxygen (O₂) and theremaining photoresist is removed by reaction with the oxygen.

However, the disk thus produced shows an unsatisfactory signal-to-noiseratio when the recording and reproduction is made after deposition ofrecording layer and protection layer on the disk. More specifically,error in the reproduced signal as well as the tracking error of theoptical beam were found to be excessive for a satisfactory recording andreproducing operation of the disk. The reason for this was studied byelectron microscopic observation of the disk and it was discovered thatdeterioration in the S/N ratio is caused by the irregularity orroughness at bottom of the groove. Such irregularity produces anunstable reflection of the optical beam. When the surface roughnessexceeds about 100 Å, the reflected optical beam becomes too unstable forsatisfactory operation of the recording and reproducing system and theproper reproduction of the information signal or proper tracking of theoptical beam is lost. At present, it is impossible to etch silica glasswithout causing irregularity at the bottom of the groove. Thus, it isnot possible to obtain disk to provide satisfactory recording andreproducing results.

SUMMARY OF THE INVENTION

Accordingly, it is a general object of the present invention to providean information recording medium and a manufacturing method thereofwherein the aforementioned problems are eliminated.

Another object of the present invention is to provide an informationrecording medium having a depression on its surface for recording aninformation signal wherein the roughness at the bottom of the depressionis minimized so that the signal-to-noise ratio of a reproduced signalreproduced from the recording medium is improved.

Another object of the present invention is to provide a method offorming a depression on a surface of a recording medium comprised of aglass material by etching, wherein the roughness at the surface of thedepression is substantially minimized so that the signal-to-noise ratioof a reproduced signal reproduced from the recording medium is improved.

Another object of the present invention is to provide an informationrecording medium having a depression on its surface for recording aninformation signal wherein the recording medium comprises a glasssubstrate of a barium borosilicate glass containing alumina and bariumoxide but free from alkalis and alkali earth elements and having anetching rate which is less than sixty percent as compared to the etchingrate of the silica glass. According to the present invention, theroughness at the bottom of the depression is successfully reduced as aresult of slow etching rate and the signal-to-noise ratio of thereproduced signal is substantially improved. Further, the recordingmedium of the present invention has a substantially negligiblebirefringence, virtually absorbs no water, and can be easilymanufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS.1(A)-(G) are schematic illustrations showing a first embodiment ofthe recording medium of the present invention as well as the steps formanufacturing the recording medium; and

FIG.2 is a perspective view showing a second embodiment of the recordingmedium of the present invention.

DETAILED DESCRIPTION

The present invention is based on a series of experiments exploring therelation between the etching rate and surface roughness at the bottom ofa depression or groove formed as a result of etching. The experimentswere conducted for various types of glasses as is summarized in Table 1and a depression or groove having width of about 2 μm and a depth ofabout 0.1-0.5 μm in correspondence with the actual depth of the grooveof the disk is formed in a spiral formation on the surface of the glassby the plasma etching technique. For a convenience of observation, thewidth of the groove (about 2 μm) is chosen to be slightly larger thanthe usual width (0.5-0.8 μm) of the groove formed on the actual disk sothat one can measure the depth of the groove precisely by a probe. Eachof the glasses were polished so that the surface roughness becomes lessthan 30 Å before the start of the experiments. The plasma etching wasachieved under a total pressure of 2.0×10⁻² Torr using CF₄ as the plasmagas and a high frequency power (RF power) of 200 watts is supplied inorder to establish the plasma.

Referring to Table I, the type II silica glass is a silica glasscontaining OH radical amounting to about 150-400 ppm and the type IIIsilica glass is a silica glass containing OH radical amounting to up to1000 ppm. The type II and type III silica glasses are commerciallyavailable glasses under the trade name of Heralux and Suprasil,respectively. The chemical composition of the glasses used in theexperiments is listed in Table I. As usual, the chemical composition isrepresented in percent by weight of the respective components in theform of oxide.

                  TABLE I    ______________________________________    sample  Heralux* Suprasil*                              #7740 plate                                         #0317 #7059    i.d.                            glass    Type    Type II  Type III boro- soda-                                         soda- barium    of      silica   silica   silicate                                    lime alumi-                                               boro-    glass   glass    glass    glass glass                                         no sili-                                               silicate                                         cate    etching 470      360      380   83   93    107    rate    (A/min)    surface***            X        X        X     O    O     O    roughness    SiO.sub.2            100      100      81    71   61    49    Na.sub.2 O                 4    15   13    K.sub.2 O                             3    MgO                             4     4    CaO                             7    Al.sub.2 O.sub.3           2    2    17    10    B.sub.2 O.sub.3           13               15    BaO                                        25    ______________________________________     *trade name     ***surface roughness     X: unacceptable (>100 Å)   -     O: substantially less than 100 Å-

From Table I, it can be seen that the type II silica glass, type IIIsilica glass and the borosilicate glass (#7740) having large etchingrates show rough surfaces having a surface roughness exceeding 100 Åwhile the soda lime glass (plate glass commonly used as a panel), sodaaluminosilicate glass (#0317) and the barium borosilicate glass (#7059)having small etching rates show smooth surfaces having a surfaceroughness substantially less than 100 Å. Thus, the etching rate and thesurface roughness are proportional to each other and as the etching rateincreases, the surface roughness increases. From Table I, it can be seenthat the glass samples which showed acceptable surface roughness wereetched with an etching rate of 83 Å/min (plate glass), 93 Å/min (#0317)and 107 Å/min (#7059). These etching rates are substantially smallerthan the etching rate for the silica glass samples and it can be safelyconcluded that an acceptable surface roughness is obtained when theetching rate is substantially less than 60% of the etching rate of thesilica glass.

From the Table I, it can be seen also that the etching rate is increasedwith increased content of SiO₂ and B₂ O₃ component and is decreased withincreased content of Na₂ O, K₂ O, MgO, CaO, Al₂ O₃ and BaO components.Generally, a satisfactory surface roughness is obtained when the contentof the Na₂ O, K₂ O, MgO, CaO, Al₂ O₃ and BaO components exceed about 10%in weight individually or in combination.

The following Table II indicates the temperatures at which the vaporpressure equals 10 Torr for various fluoride species. These fluoridesare formed as a result of reaction between the plasma gas and the oxidecomponents in the glass. Thus, the SiO₂ component in the glass reactswith the CF₄ plasma gas to form the SiF₄ component. Similarly, the Na₂O, K₂ O, Al₂ O₃ and B₂ O₃ components in the glass form the NaF, KF, AlF₃and BF₃ components in the plasma gas as the product of reaction. Thetemperatures listed in Table II characterize the thermodynamicproperties of the components in the glass and can be regarded as acharacteristic temperature characterizing the thermodynamic properties.It is noted that the SiO₂ and B₂ O₃ components have relatively lowcharacteristic temperatures while the Na₂ O, K₂ O and Al₂ O₃ haverelatively high characteristic temperatures.

                  TABLE II    ______________________________________    Temperature at which the vapor pressure of the    fluoride component reaches 10 Torr    Fluoride            Temperature    ______________________________________    SiF.sub.4           -130.4    NaF                  1240    KF                   1039    AlF.sub.3            1324    BF.sub.3            -141.3    ______________________________________

The low characteristic temperature indicates that the correspondingoxide component in the glass reacts fast with the plasma gas and theetching rate is high. On the other hand, the high characteristictemperature indicates that the corresponding oxide component in theglass reacts slow with the plasma gas and the etching rate is decreased.Thus, the observation in Table I that the increase in the SiO₂ and B₂ O₃components in the glass increases the etching rate and the increase inthe Na₂ O, K₂ O, MgO, CaO, Al₂ O₃ and BaO components decreases theetching rate is supported by thermodynamic consideration. In the sample#7059, the B₂ O₃ component is comparable to that of the sample #7740,however, the effect of the B₂ O₃ component in the sample #7059 isconsidered to be masked by the existence of the Al₂ O₃ and BaOcomponents and further by the decrease of the SiO₂ component.

In the experiments described heretofore, the soda lime glass and thesoda aluminosilicate glass which are the glasses commonly used forgeneral purpose also falls in the preferable range as far as the etchingrate and surface roughness are concerned. However, such glasses containalkalis such as Na₂ O and K₂ O as well as the alkali earth elements suchas MgO and CaO which tend to cause corrosion or pinholes in therecording layer of the optical and magneto-optical disk. Thus, theseglasses are not eligible for the substrate of the disk. In other words,the glass material to be used for the substrate of the optical andmagneto-optical disks should not only show the etching rate less than60% of the etching rate of the silica glass but should also be free fromalkali which include one or more elements selected from the groupconsisting of sodium, magnesium, potassium, and calcium. In the opticaldisk of the present invention to be described, the barium borosilicateglass containing alumina and barium oxide (#7059) is used for thesubstrate of the disk. This glass contains the Al₂ O₃ and BaO componentsin more than 10 and 25 percent by weight respectively. Note that thetotal content of Na₂ O, K₂ O, MgO and CaO in sample No. 7059 is lessthan about 1 wt. % as shown in Table I.

As the Na₂ O, K₂ O, MgO and CaO components are undesirable in theconstituent of the glass because of the corrosion and pinhole formationas previously described, the glass to be used for the substrate of thedisk should be the one containing Al₂ O₃ and BaO components amounting tomore than 10 percent by weight individually or in combination. At thesame time, such glass should show an etching rate which is less than 60%of the etching rate of the silica glass.

Next, manufacturing of the disk will be described with reference toFIGS. 1(A)-(G) which show a series of manufacturing steps of a disk 10which is illustrated in the completed form in FIG.1(G). In the presentembodiment, the disk 10 is an erasable optical disk having a TeOxrecording layer which changes the reflectivity responsive to therecording made by irradiation of the optical beam. However, the presentinvention is by no means limited to this particular type of the disk butmay be applicable to other optical and magneto-optical type disks ingeneral.

Referring to FIG. 1(G), the optical disk 10 comprises a disk-shapedsubstrate 16 bounded by a flat bottom 16a, a TeOx recording layer 18deposited on a surface 16b of the substrate 16 and a protection layer 19formed on the recording layer 18. The protection layer 19 may betransparent or may be opaque and may be bounded by a flat surface 19a ormay be bounded by a surface 19a which is not substantially flat. Thesubstrate 16 is made of the #7059 glass in Table I which is transparentto an energy beam used for recording and reproducing the informationsignal on and from the recording layer 18 and having the etching rateless than 60% of the etching rate of silica glass. The substrate 16 isformed with grooves 13 having a concentric or spiral-shaped pattern onits surface 16b and the surface 16b of the substrate is covered by arecording layer 18 of TeOx. TeOx is a non-stoichiometric compound of Teand TeO₂ and causes a phase transition responsive to the heating by arelatively intense optical beam. Responsive to the phase transition, thereflectivity of the TeOx layer 18 is changed and the recording ofinformation signal is achieved as a change in the reflectivity of therecording layer 18. The recorded information is erased by annealing theTeOx layer 18 by a relatively low energy optical beam. TeOx may be dopedwith Ge and Sn. The principle of the erasable recording system usingTeOx as the recording medium is well known and no further descriptionwill be given. The guide groove has a width of less than 1 μmcorresponding to the beam spot of the optical beam used for therecording and reproducing of the information signal as is usual in theart. Similarly, the depth of the groove is less than 0.1 μm as is usual.

Referring to FIG. 1(A), the substrate 16 is first applied with aphotoresist 12 with a uniform thickness. The photoresist 12 may be anyphotoresist commonly used in the patterning of semiconductor chips.Then, a focused laser beam 17 is irradiated on the surface of thephotoresist continuously while revolving the substrate 16 and thephotoresist 12 unitarily around a central axis of the substrate as shownin FIG. 1(B). At the same time, the laser beam 17 is scanned on thesurface of the photoresist 12 and a spiral-shaped pattern is drawn onthe surface of the photoresist 12 by the laser beam. The spot of thelaser beam on the surface of the photoresist, the seed of revolution ofthe substrate 16, and the speed of scanning of the laser beam are chosensuch that a predetermined spiral pattern is written with a predeterminedthickness. If the laser beam is intensity-modulated, a series of pitswill be recorded accordingly along the predetermined spiral pattern.

Next, the photoresist 12 is developed as shown in FIG. 1(C) wherein aportion of the photoresist 12 irradiated with the laser beam is removed.In other words, the portion of the substrate corresponding to theportion of the photoresist irradiated with the laser beam 17 is exposed.

The substrate 16 having a reminder of the developed photoresist is thenbrought into a reaction chamber of a plasma etching apparatus (notshown). The reaction chamber is supplied with a CF₄ gas and the exposedportion of substrate is subjected to dry etching in a plasma 14 of CF₄shown in FIG. 1(D). During this plasma etching, the etching rate iscontrolled so that the etching rate is less than 60% of the etching rateof the silica glass and a spiral groove 13' is formed. The groove 13'corresponds to the guide groove 13 of the disk. The groove 13' thusformed has a bottom surface 13a which is sufficiently smooth and thesurface roughness of the surface 13a is limited to substantially below100 Å. The etching is continued until the groove 13' reaches apredetermined depth as shown in FIG. 1(D). The plasma etching apparatusis well known apparatus commonly used in the manufacturing of thesemiconductor integrated circuit chips and the description thereof willbe omitted.

Next, the gas supplied to the reaction chamber is changed from the CF₄gas to an O₂ and the remaining photoresist 12 is removed by reactionwith the plasma gas now containing O₂. This procedure is called ashing.As a result, the substrate 16 is formed with the groove 13' is obtainedas shown in FIG. 1(E).

The substrate 16 thus obtained is then deposited with a layer 18 of TeOxcompound by sputtering as shown in FIG. 18(F). The thickness of thelayer 18 is usually about 0.1 μm and the guide groove 13 is formed on asurface of the layer 18. Then the protection layer 19 is applied on thelayer 18 by applying a resin such as the UV-cured resin or by depositingSiO₂, Si₃ N₄, SiC or C on the surface of the layer 18 by sputtering orvacuum evaporation. Thus, the disk 10 as shown in FIG. 1(G) is obtained.When the protection layer 19 is formed by the resin, the surface 19a maybe generally flat. On the other hand, when the protection layer 19 isprovided by sputtering or vacuum evaporation, the surface 19a is formedwith a pattern corresponding to the groove 13 on the recording layer.Because the reduced surface roughness at the bottom of the guide grooveat which the incident optical beam is reflected, the S/N ratio in theoptical beam reflected back from the guide groove of such disk isimproved and the recording and reproduction of the information signal onand from the disk is performed satisfactorily.

Further, the disk 10 thus obtained is subjected to an environmental testfor reliability. The disk 10 is held at a temperature of 60° C. andrelative humidity of 90% for a period exceeding 1000 hours. In thistest, no appearance of corrosion or pinholes was observed. Thus, thedisk of the present invention can be stored without problem for aprolonged period of time.

Although the manufacturing of the disk 10 used the laser beam 17 forwriting the pattern of the guide groove 13 on the photoresist 12, thepatterning of the guide groove 13 on the photoresist 12 may be achievedby irradiation of ultraviolet light using a photomask formed with apattern of the guide groove, 13, or by using an electron beam projectedon the layer of photoresist in the vacuum chamber.

Further, the gas used for the plasma etching is not limited to the CF₄gas but may be any gas containing fluoride vapor such as CHF₃ as far asthey react with the glass. For example, the plasma gas may contain C₂F₆, C₃ F₈, NF₃ and a mixture thereof.

FIG. 2 shows a second embodiment of the disk of the present invention.In the drawing, those portions corresponding to the portion alreadyillustrated in FIGS. 1(A)-(G) are given identical reference numerals andthe description thereof will be omitted. In order to facilitate theunderstanding of the drawing, the protection layer 19 is not illustratedin FIG. 2. Referring to the drawing, the guide groove 13 is defined by apair of tapered portion 13b which extends along the groove 13 at theboth sides which also subject the reflection of the optical beam, thusthe tracking of the optical beam is improved. Such a tapered portion maybe formed according to a method disclosed in the aforementioned U.S.Pat. No. 4,655,876.

Further, the guide groove is not limited to the spiral groove but may bea plurality of concentric formation.

Further, the present invention in not limited to the embodimentdescribed heretofore, but various variations and modification may bemade without departing from the scope of the invention.

What is claimed is:
 1. An information recording disk for recording aninformation signal along a track defined by a depression on the disk asa change of physical property of a recording material deposited on thetrack comprising:a disk-shaped glass substrate made of a glass definedby a flat first surface at a first side and a second surface carrying agroove corresponding to the track at a second side opposite to the firstside, said groove being defined by a bottom surface having a surfaceroughness substantially smaller as compared to the surface roughnesscaused at a bottom surface of a groove on a silica glass substrate whenboth the disk-shaped glass substrate and silica glass substrate aredry-etched under same conditions, and said disk-shaped glass substratecomprising SiO₂ component and one or both of Al₂ O₃ and BaO componentswhile free from those alkali and alkali earth elements having one ormore elements selected from a group consisting of sodium, magnesium,potassium, and calcium; a recording layer comprised of the recordingmaterial which changes in physical property responsive to a projectedenergy beam modulated with the information signal, said recording layerdeposited on the second surface of the substrate having one side thereofmaking an intimate contact with the second surface, said recording layercarrying said depression which defines the track on other side of therecording layer; and a protection layer deposited on said another sideof the recording layer so that the depression on the recording layer isburied under the protection layer with an intimate contact between theprotection layer and the recording layer.
 2. An information recordingdisk as claimed in claim 1 in which the surface roughness at the bottomsurface of the groove is substantially smaller than 100 Å.
 3. Aninformation recording disk as claimed in claim 1 in which thedisk-shaped glass substrate comprises a glass which shows an etchingrate substantially smaller than 60% of the etching rate of silica glasswhen both the glass and the silica glass are dry etched under sameconditions.
 4. An information recording disk as claimed in claim 1 inwhich at least one of the Al₂ O₃ and BaO components int he glass iscontained by an amount equal to or larger than 10% by weight.
 5. Aninformation recording disk as claimed in claim 1 in which the Al₂ O₃component is contained in the glass by an amount equal to or larger than10 percent by weight.
 6. An information recording disk as claimed inclaim 1 in which the BaO component is contained in the glass by anamount equal to or larger than 10 percent by weight.
 7. An informationrecording disk as claimed in claim 1 in which the BaO component iscontained in the glass by an amount equal to or larger than 25 percentby weight.
 8. An information recording disk as claimed in claim 1 inwhich the Al₂ O₃ and BaO components are contained in the glass by anamount such that the total content of the Al₂ O₃ and BaO components arecontained in the glass by an amount such that the total content of theAl₂ O₃ and BaO components is equal to or larger than 10 percent byweight.
 9. An information recording disk as claimed in claim 1 in whichthe depression on said another side of the recording layer is a groovewhich is defined at both sides thereof by a pair of declined surfacesextending along the depression.
 10. An information recording disk asclaimed in claim 1 in which the recording layer is a layer of TeOxcompound.
 11. A method of manufacturing an information recording diskfor recording an information signal along a track defined by adepression as a change of physical property of a recording materialdeposited on the depression comprising steps of:applying a photoresiston a surface of a disk-shaped glass substrate made of a glass free fromthose alkali and alkali earth elements having one or more elementsselected from a group consisting of sodium, magnesium, potassium, andcalcium; exposing the photoresist to a radiation of energy beam so as towrite a pattern on the photoresist by the radiation, developing thephotoresist and removing a portion of the photoresist irradiated by theradiation from the surface of the glass substrate; subjecting the glasssubstrate to an etching at an etching rate substantially slower than anetching rate which is obtained when a silica glass is etched under asame etching condition and forming a groove deep into the glasssubstrate at a portion of the surface of the glass substrate which isnot covered by the photoresist after the etching, said groove formed bythe dry etching being corresponding with the depressions defining saidtrack removing a remaining photoresist from the surface of the glasssubstrate; depositing the recording material, physical property thereofis transformable responsive to further radiation of energy beammodulated with the information signal, on the surface of the glasssubstrate to form a recording layer such that the depression whichdefines the track is formed on a surface of the recording layer incorrespondence with the groove at the surface of the glass substrate;and depositing a protection layer on the surface of the recording layerfor protecting the depression at the surface of the recording layer. 12.A method of manufacturing an information recording disk as claimed inclaim 11 in which said step of etching comprises a dry etching of theglass at an etching rate substantially slower than 60 percent of theetching rate which is obtained when the silica glass is etched under thesame etching condition.
 13. A method of manufacturing an informationrecording disk as claimed in claim 11 in which the glass substrate is aglass comprising SiO₂ component and one or both of Al₂ O₃ and BaOcomponents such that at least one of the Al₂ O₃ and BaO components iscontained by an amount equal to or larger than 10% by weight.
 14. Amethod of manufacturing an information recording disk as claimed inclaim 11 in which the glass substrate is a glass comprising SiO₂component and Al₂ O₃ and BaO components such that the total amount ofthe Al₂ O₃ and BaO components in the glass is equal to or larger than10% by weight.
 15. A method of manufacturing an information recordingdisk as claimed in claim 11 in which the step of exposing thephotoresist to radiation is performed by a laser beam.
 16. A method ofmanufacturing an information recording disk as claimed in claim 11 inwhich the step of exposing the photoresist to radiation is performed byusing a photomask carrying a pattern corresponding to the groove to beformed on the glass substrate.
 17. A method of manufacturing aninformation recording disk as claimed in claim 12 in which the step ofetching comprises plasma etching of the glass substrate using a plasmagas containing one or more fluoride species selected from a group ofCF₄, CHF₃, C₂ F₆, C₃ F₈ and NF₃.
 18. A method of manufacturing aninformation recording disk as claimed in claim 12 in which the etchingrate of the glass substrate is chosen substantially smaller than 360Å/min.
 19. A method of manufacturing an information recording disk asclaimed in claim 12 in which the etching rate of the glass substrate ischosen smaller than 107 Å/min.
 20. An information recording disk asclaimed in claim 1 in which said glass substrate is free of sodium andpotassium.